1. Field of the Invention
The present invention relates to a driver circuit for a semiconductor laser diode (hereafter denoted as LD), in particular, the inventions relates to a driver circuit with a shunt-driving configuration.
2. Related Background Art
The shunt driver for the LD has been well known as one type of the driver circuit. For instance, U.S. Pat. No. 5,513,197, has disclosed the laser driver circuit with the shunt-driving configuration. The shunt-driving configuration provides a switching transistor connected in parallel to the LD and shunts the bias current flowing in the LD by this switching transistor to modulate the LD. In a practical application for the LD such as an optical transmitter installed in the optical communication, two conditions are inevitable, that is, the average output power and the extinction ratio of the light emitted from the LD must be constant independent of both internal and external conditions. The LD installed within the optical transmitter shows large temperature dependence, that is, the threshold current increases and the slope efficiency decreases at a high temperature. Accordingly, an ordinary optical transmitted installs, what is called, the automatic power control (hereafter denoted as APC) to keep the output power and the extinction ration independent of the temperature. The APC circuit provides a larger bias current to the LD as the temperature thereof increases.
The shunt-driver circuit may adjust the bias current supplied to the LD and the amplitude of the driving signal for the switching transistor to keep the average power and the extinction ratio of the optical output from the LD independent of the temperature. Specifically, the shunt-driver circuit increases the bias current and the amplitude of the driving signal at a high temperature, while, the circuit decreases them at a lower temperature. However, a conventional shunt-driver circuit somewhat bounds the bias condition at a high temperature even in a low temperature; accordingly, unnecessary idle current is supplied to the LD or the switching transistor connected in parallel to the LD is left in a lower temperature, which becomes hard to reduce the power consumption.